Oscillators are used in many electronics circuits to provide an AC signal having a predetermined frequency. The oscillator signal may be used as a reference signal, and often it is used as one of the input signals to a mixer, where it will be combined with incoming signals at a different frequency, with the sum or difference of the two input signals being derived as an output signal. A common circumstance is when high frequency radar signals are mixed with oscillator signals which are usually of a lower frequency so as to produce an intermediate signal frequency signal which may be considerably lower in frequency.
It is sometimes required to provide an oscillator which may have a varying frequency output over, generally, a fairly limited range of frequencies. This may be useful, for example, to provide a swept input signal to a mixer to determine the presence of a higher frequency signal which may, itself, be within a specified range of frequencies and to output an intermediate frequency signal in the presence of such an input signal having been detected. For example, an incoming radar signal might be anywhere in the range of, say, from 33.4 GHz to 36.0 GHz, where the incoming radar signal could be present anywhere within that band and it is necessary to sweep a local oscillator frequency over a range of frequencies in order to develop an intermediate frequency resultant signal within a specified frequency band where the intermediate frequency signal may be further handled and analyzed.
Voltage controlled push-push oscillators have been known for some period of time, particularly because they provide a proven design that has twice the output frequency capability of any single-transistor microwave voltage controlled oscillators. This is because the output frequency of a push-push transistor voltage controlled oscillator is twice the operating frequency of either of the transistors that are present in the circuit, where the transistors operate in an anti-phase or push-push mode and produce in phase or push-push second harmonics. The output frequency is extracted from the circuit at a point of fundamental null and second harmonic maxima, and thus has a frequency that is twice the frequency at which each of the transistors oscillates. However, until now, push-push transistor voltage controlled oscillators have generally operated in a configuration where the collectors of each of the pair of transistors are grounded, the emitters of the transistors are connected through a capacitance to ground in each instance, and the bases of the transistors are connected through inductance elements to each other with the output frequency from the oscillator being extracted at a balance point between the bases.
In contradistinction to the prior art, the present invention provides a push-push oscillator which is distinguished by the fact that the transistors may be connected to each other in a common collector configuration, and in any event are connected so as to be AC grounded at the output frequency of the oscillator. Although the push-push oscillator of the present invention is capable of operating at a frequency which may be chosen over a very broad range of frequencies, it generally is intended to operate in the microwave frequency so as to have an output frequency that may be in the range of from about 9 GHz up to about 18 GHz. Because the push-push oscillator is a voltage controlled oscillator, its output will have a particular bandwidth, and usually when the oscillator is working in the microwave frequency range the bandwidth would be about 3.0 GHz--i.e., 1.5 GHz.+-.based on a centre frequency. When such a push-push voltage controlled oscillator is utilized in a commercial embodiment such as a radar detector, it is usual that bipolar transistors are preferred because they have better second harmonic characteristics due to inherent non-linearities.
The principle embodied in any push-push oscillator, of using two transistors to generate harmonics and then cancelling out the fundamental frequency at which either of the transistors operates, is well known, as noted above. However, quite unexpectedly, the present inventor has determined that a better and more efficient push-push oscillator may be obtained, giving among other advantages the fact that the output frequency may be extended to a higher frequency of operation for similar devices than that of previous designs, because the emitters of the pair of transistors used in the push-push oscillator are connected to the resonator element which is included in the push-push oscillator design. Moreover, while the resonator element in general may comprise a bisected resonator element that has a pair of half-elements that are arranged such that an output tap is placed between the bisected half-elements. At that point, there will exist an RF null at the oscillating frequency of each of the pair of transistors, and an anti-null at the output frequency of the oscillator. Moreover, it has been found to be particularly advantageous to utilize a microstrip resonator as the resonant element. In that case, as discussed in greater detail hereafter, the microstrip itself is bisected in a manner so as to have an output tap at which the output frequency of the push-push oscillator is derived. At the same point, the fundamental or oscillating frequency of each of the transistors is cancelled, thereby creating a null at that frequency.
In order to drive the push-push oscillator, it is necessary to provide a DC source of voltage which, in the present invention, is connected directly to the commonly connected collectors; a source of DC current which is connected to the output tap of the resonator element or elements and thus to each of the emitters of each of the pair of transistors; and a source of variable DC tuning voltage which is connected to each of the bases of the pair of transistors.
Certain other advantages are derived from the present invention, particularly when bipolar transistors are connected in the common collector configuration and the resonator element is a microstrip resonator. They include the fact that bipolar transistor circuits having an inductive collector to base impedance present the resonator with a complex impedance, which will have a negative real component in a frequency domain around the oscillating frequency of each transistor. Moreover, the design of a microstrip resonator element may be arranged so that it has a length which is at least one-half the wavelength of the highest oscillating frequency at which each transistor is intended to be driven plus enough extra length at each end of the microstrip resonator element which is remote from the output tap, so as to provide a capacitive reactance whose function is to contribute to the resonant circuit whereby the oscillating frequency of each of the respective transistors in the push-push oscillator may be governed. This comes, of course, from the fact that a resonant circuit may generally be defined as being an LRC circuit, utilizing an inductance, resistance (or negative resistance), and capacitance in the well known manner.
However, the present invention provides circuits whereby the negative resistance between the emitter and collector of each of the transistors utilized in the circuit may be obtained or achieved by converting the collector to base reactance of each transistor, which is capacitive in nature, to become net inductive at the oscillating frequency of each transistor. This results in an inductive reactance appearing in parallel with a negative resistance. This, in turn, permits the emitter of each of the transistors utilized in the push-push configuration of voltage controlled oscillator according to the present invention to provide return gain (as seen from the emitter port) at frequencies that are in range of and may even exceed the transition frequency of the transistors being utilized.
Thus, the present invention is contrary to usual practice where emitter output would not be utilized for derivation of output frequency from each transistor, since biasing is more difficult because, from a DC point of view, there would be less feedback to ensure that each transistor is operated at the same current if the emitters are DC connected through the resonator, thus requiring transistors with matched DC parameters. The present invention does, however, utilize emitter output where the emitters of the push-push oscillator are connected to the resonator in such a manner that an output tap will exist between the resonator elements--which may be lumped elements or distributed reactances--and where the output tap is derived at a point between the lumped elements or midway along a distributed resonator element such that there will be an RF null at the oscillating frequency of each of the pair of transistors, and there will be an anti-null at the output frequency of the oscillator.
It must not be overlooked, of course, that any push-push oscillator must rely on the generation of signals from each transistor that are rich in second harmonic components because the fundamental components will cancel since the transistors operate in a push-pull mode at the fundamental, and therefore it is the second harmonic component which must be present and which gives the output frequency at twice the fundamental or oscillating frequency of each of the transistors. However, in the prior an where push-push voltage controlled oscillators were configured having resonators connected to the bases of each transistor, then saturation of the transistors was relied upon in order to generate the rich second harmonic component. On the other hand, in keeping with the present invention, if the resonator element is connected to emitter of each of the transistors that is used in the voltage controlled oscillator, then transistor cut-off is the principal characteristic that is relied upon so as to generate the non-linearity. This, in turn, allows for much higher frequency of operation, since transistor cut-off switching does not reduce the frequency response of the transistors as it does with transistor saturation.